Direct SW aerosol radiative forcing over Portugal

The estimation of radiative forcing due to desert dust and forest fires aerosols is a very important issue since these particles are very efficient at scattering and absorbing both short and longwave radiation. In this work, the evaluation of the aerosol radiative forcing at the top of the atmosphere over the south of Portugal is made, particularly in 5 the regions ofÉvora and of Cabo da Roca. The radiative transfer calculations combine ground-based and satellite measurements , to estimate the top of the atmosphere direct SW aerosol radiative forcing. The method developed to retrieve the surface spectral reflectance is also presented, based on ground-based measurements of the aerosol optical properties combined with the 10 satellite-measured radiances. The aerosol direct radiative effect is shown to be very sensitive to the underlying surface , since different surface spectral reflectance values may originate different forcing values. The results obtained also illustrate the importance of considering the actual aerosol properties, in this case measured by ground-based instrumentation, particu-15 larly the aerosol single scattering albedo, because different aerosol single scattering albedo values can flip the sign of the direct SW aerosol radiative forcing. The instantaneous direct SW aerosol radiative forcing values obtained at the top of the atmosphere are, in the majority of the cases, negative, indicating a tendency for cooling the Earth. For Desert Dust aerosols, overÉvora land region, the average 20 forcing efficiency is estimated to be −25 W/m 2 /AOT 0.55 whereas for Cabo da Roca area, the average forcing efficiency is −46 W/m 2 /AOT 0.55. In the presence of Forest Fire aerosols, over Cabo da Roca region, the average value of forcing efficiency is −28 W/m 2 /AOT 0.55 and overÉvora region an average value of −33 W/m 2 /AOT 0.55 is found.